Floating gate flash memory architecture includes several critical implant layers, which are referred to as middle of line (MOL) implant layers. Referring to FIG. 1, an exemplary middle-of-line (MOL) implant layer 100 of a flash memory device is shown. As is known in the art, MOL implant layers can include numerous trenches 102, which can create a severe topography in a surface of the MOL implant layer. Implantation of the MOL implant layer 100 is performed through openings 106 formed in a resist layer 108, which is deposited over the MOL implant layer 100. Conventionally, positive tone resists are used to protect a top surface 104 of the MOL implant layer, while trench regions are exposed for implantation.
With further reference to FIG. 2A and FIG. 2B, conventional processing steps for forming the resist layer 108 on the MOL implant layer are shown. The trenches 102 are formed initially using conventional techniques. Next, the resist layer 108 is deposited generally over the MOL layer 100, covering the MOL implant layer 100 and filling the trenches 102. Using conventional photolithographic techniques, the resist layer 108 is selectively exposed using optical radiation 110 and a mask 112. The exposed resist subsequently is developed to remove a portion of the resist, e.g., the exposed portion for a positive tone resist. With respect to the MOL implant layer, the desired result is to remove the resist 108 from each trench 102, while retaining the resist along the top surface 104 of the MOL implant layer.
A pervasive trend in modern integrated circuit manufacture is to produce semiconductor devices, such as flash memory devices, that are as small as possible. The reduction in size of the flash memory device results in a surface of the wafer having a severe topography, particularly at the MOL implant layers 100. Additionally, the MOL implant layers tend to be highly reflective, creating problems with CD control. Due in part to the severe topography of the MOL implant layer, some resist residue 114 inevitably remains in the trenches 102 after the resist has been exposed and developed. The residue or “scumming” as it is referred to in the art is difficult to remove from the trenches and can lead to problems during subsequent implantation steps. For example, the residue can block implants and/or cause implant non-uniformity, which can increase the potential for defective memory devices and thus lower yields.
Accordingly, it is desirable that a bottom portion 116 of each trench 102 be free of resist residue prior to implantation. A conventional method of removing resist residue from the trench regions is through an oxygen plasma descumming step, which is performed after the resist has been exposed and developed. In oxygen plasma descumming, the wafer is placed in a chamber having an oxygen plasma atmosphere. The oxygen plasma causes a chemical reaction with surface contaminants on the wafer, e.g., the residue in the trenches, resulting in their volatilization and subsequent removal from the plasma chamber.
A drawback to oxygen plasma descumming is that the resist image on the wafer can be affected by the descum step. For example, during the oxygen plasma descumming step, the resist sidewalls can be partially removed by the oxygen plasma, thereby altering or enlarging openings in the resist layer. Since the resist pattern is altered from its intended image, overlay margin, for example, can be affected.
Accordingly, there exists a need in the art for a method of fabricating MOL implant layers in flash memory devices that minimizes the amount of resist residue left in the trenches of the MOL implant layers. Additionally, it would be advantageous if the descumming step could be eliminated altogether.